I have mentored in this way before. I mentored on Codementor (codementor.io) for a bit. The technology to share code and discuss was ok. Easily good enough to express ideas and help a mentee.
The difficult bit wasn't really the mentoring. It was the finding and connecting with mentees.
There is a tendancy with such sites for a race to the bottom. A lot of people just want you to do some job on the cheap or to help them with their homework. Having said that, I have kept in touch with a few of my mentees.
I did a similar thing for a charity where I had a remote conversation every week with someone hoping to become a junior developer. I found that rewarding and again the learning process worked well enough with video conferencing, code sharing and github.
This was before Live Share in VS Code. The options for live code share were limited. Live Share would have made the process easier and I've used that in remote pairing situations. Works well.
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